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Pointed the Right Way

story by john hagerman

Camber, Caster and Toe:
What Do They Mean?

The three major alignment parameters on a
car are toe, camber, and caster. Most enthusiasts have a good understanding of what these
settings are and what they involve, but many may not know why a particular setting is
called for, or how it affects performance. Let's take a quick look at this basic aspect of
suspension tuning.

UNDERSTANDING TOE

When a pair of wheels is set so that their
leading edges are pointed slightly towards each other, the wheel pair is said to have
toe-in. If the leading edges point away from each other, the pair is said to have toe-out.
The amount of toe can be expressed in degrees as the angle to which the wheels are out of
parallel, or more commonly, as the difference between the track widths as measured at the
leading and trailing edges of the tires or wheels. Toe settings affect three major areas
of performance: tire wear, straight-line stability and corner entry handling
characteristics.

For minimum tire wear and power loss, the
wheels on a given axle of a car should point directly ahead when the car is running in a
straight line. Excessive toe-in or toe-out causes the tires to scrub, since they are
always turned relative to the direction of travel. Too much toe-in causes accelerated wear
at the outboard edges of the tires, while too much toe-out causes wear at the inboard
edges.

So if minimum tire wear and power loss are
achieved with zero toe, why have any toe angles at all? The answer is that toe settings
have a major impact on directional stability. The illustrations at right show the
mechanisms involved. With the steering wheel centered, toe-in causes the wheels to tend to
roll along paths that intersect each other. Under this condition, the wheels are at odds
with each other, and no turn results.

When the wheel on one side of the car
encounters a disturbance, that wheel is pulled rearward about its steering axis. This
action also pulls the other wheel in the same steering direction. If it's a minor
disturbance, the disturbed wheel will steer only a small amount, perhaps so that it's
rolling straight ahead instead of toed-in slightly. But note that with this slight
steering input, the rolling paths of the wheels still don't describe a turn. The wheels
have absorbed the irregularity without significantly changing the direction of the
vehicle. In this way, toe-in enhances straight-line stability.

If the car is set up with toe-out,
however, the front wheels are aligned so that slight disturbances cause the wheel pair to
assume rolling directions that do describe a turn. Any minute steering angle beyond the
perfectly centered position will cause the inner wheel to steer in a tighter turn radius
than the outer wheel. Thus, the car will always be trying to enter a turn, rather than
maintaining a straight line of travel. So it's clear that toe-out encourages the
initiation of a turn, while toe-in discourages it.

With toe-in (left) a
deflection of the suspension does not cause the wheels to initiate a turn as with toe-out
(right).

The toe setting on a particular car
becomes a tradeoff between the straight-line stability afforded by toe-in and the quick
steering response promoted by toe-out. Nobody wants their street car to constantly wander
over tar strips-the never-ending steering corrections required would drive anyone batty.
But racers are willing to sacrifice a bit of stability on the straightaway for a sharper
turn-in to the corners. So street cars are generally set up with toe-in, while race cars
are often set up with toe-out.

With four-wheel independent suspension,
the toe must also be set at the rear of the car. Toe settings at the rear have essentially
the same effect on wear, directional stability and turn-in as they do on the front.
However, it is rare to set up a rear-drive race car toed out in the rear, since doing so
causes excessive oversteer, particularly when power is applied. Front-wheel-drive race
cars, on the other hand, are often set up with a bit of toe-out, as this induces a bit of
oversteer to counteract the greater tendency of front-wheel-drive cars to understeer.

Remember also that toe will change
slightly from a static situation to a dynamic one. This is is most noticeable on a
front-wheel-drive car or independently-suspended rear-drive car. When driving torque is
applied to the wheels, they pull themselves forward and try to create toe-in. This is
another reason why many front-drivers are set up with toe-out in the front. Likewise, when
pushed down the road, a non-driven wheel will tend to toe itself out. This is most
noticeable in rear-drive cars.

The amount of toe-in or toe-out dialed
into a given car is dependent on the compliance of the suspension and the desired handling
characteristics. To improve ride quality, street cars are equipped with relatively soft
rubber bushings at their suspension links, and thus the links move a fair amount when they
are loaded. Race cars, in contrast, are fitted with steel spherical bearings or very hard
urethane, metal or plastic bushings to provide optimum rigidity and control of suspension
links. Thus, a street car requires a greater static toe-in than does a race car, so as to
avoid the condition wherein bushing compliance allows the wheels to assume a toe-out
condition.

It should be noted that in recent years,
designers have been using bushing compliance in street cars to their advantage. To
maximize transient response, it is desirable to use a little toe-in at the rear to hasten
the generation of slip angles and thus cornering forces in the rear tires. By allowing a
bit of compliance in the front lateral links of an A-arm type suspension, the rear axle
will toe-in when the car enters a hard corner; on a straightaway where no cornering loads
are present, the bushings remain undistorted and allow the toe to be set to an angle that
enhances tire wear and stability characteristics. Such a design is a type of passive
four-wheel steering system.

THE EFFECTS OF CASTER

Caster is the angle to which the steering
pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the
pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than
the bottom pivot), then the caster is positive; if it's tilted forward, then the caster is
negative.

Positive caster tends to straighten the
wheel when the vehicle is traveling forward, and thus is used to enhance straight-line
stability. The mechanism that causes this tendency is clearly illustrated by the castering
front wheels of a shopping cart (above). The steering axis of a shopping cart wheel is set
forward of where the wheel contacts the ground. As the cart is pushed forward, the
steering axis pulls the wheel along, and since the wheel drags along the ground, it falls
directly in line behind the steering axis. The force that causes the wheel to follow the
steering axis is proportional to the distance between the steering axis and the
wheel-to-ground contact patch-the greater the distance, the greater the force. This
distance is referred to as "trail."

Due to many design considerations, it is
desirable to have the steering axis of a car's wheel right at the wheel hub. If the
steering axis were to be set vertical with this layout, the axis would be coincident with
the tire contact patch. The trail would be zero, and no castering would be generated. The
wheel would be essentially free to spin about the patch (actually, the tire itself
generates a bit of a castering effect due to a phenomenon known as "pneumatic
trail," but this effect is much smaller than that created by mechanical castering, so
we'll ignore it here). Fortunately, it is possible to create castering by tilting the
steering axis in the positive direction. With such an arrangement, the steering axis
intersects the ground at a point in front of the tire contact patch, and thus the same
effect as seen in the shopping cart casters is achieved.

The tilted steering axis has another
important effect on suspension geometry. Since the wheel rotates about a tilted axis, the
wheel gains camber as it is turned. This effect is best visualized by imagining the
unrealistically extreme case where the steering axis would be horizontal-as the steering
wheel is turned, the road wheel would simply change camber rather than direction. This
effect causes the outside wheel in a turn to gain negative camber, while the inside wheel
gains positive camber. These camber changes are generally favorable for cornering,
although it is possible to overdo it.

Most cars are not particularly sensitive
to caster settings. Nevertheless, it is important to ensure that the caster is the same on
both sides of the car to avoid the tendency to pull to one side. While greater caster
angles serve to improve straight-line stability, they also cause an increase in steering
effort. Three to five degrees of positive caster is the typical range of settings, with
lower angles being used on heavier vehicles to keep the steering effort reasonable.

Like a shopping cart wheel
(left) the trail created by the castering of the steering axis pulls the wheels in line.

WHAT IS CAMBER?

Camber is the angle of the wheel relative
to vertical, as viewed from the front or the rear of the car. If the wheel leans in
towards the chassis, it has negative camber; if it leans away from the car, it has
positive camber (see next page). The cornering force that a tire can develop is highly
dependent on its angle relative to the road surface, and so wheel camber has a major
effect on the road holding of a car. It's interesting to note that a tire develops its
maximum cornering force at a small negative camber angle, typically around neg. 1/2
degree. This fact is due to the contribution of camber thrust, which is an additional
lateral force generated by elastic deformation as the tread rubber pulls through the
tire/road interface (the contact patch).

To optimize a tire's performance in a
corner, it's the job of the suspension designer to assume that the tire is always
operating at a slightly negative camber angle. This can be a very difficult task, since,
as the chassis rolls in a corner, the suspension must deflect vertically some distance.
Since the wheel is connected to the chassis by several links which must rotate to allow
for the wheel deflection, the wheel can be subject to large camber changes as the
suspension moves up and down. For this reason, the more the wheel must deflect from its
static position, the more difficult it is to maintain an ideal camber angle. Thus, the
relatively large wheel travel and soft roll stiffness needed to provide a smooth ride in
passenger cars presents a difficult design challenge, while the small wheel travel and
high roll stiffness inherent in racing cars reduces the engineer's headaches.

It's important to draw the distinction
between camber relative to the road, and camber relative to the chassis. To maintain the
ideal camber relative to the road, the suspension must be designed so that wheel camber
relative to the chassis becomes increasingly negative as the suspension deflects upward.
The illustration on the bottom of page 46 shows why this is so. If the suspension were
designed so as to maintain no camber change relative to the chassis, then body roll would
induce positive camber of the wheel relative to the road. Thus, to negate the effect of
body roll, the suspension must be designed so that it pulls in the top of the wheel (i.e.,
gains negative camber) as it is deflected upwards.

While maintaining the ideal camber angle
throughout the suspension travel assures that the tire is operating at peak efficiency,
designers often configure the front suspensions of passenger cars so that the wheels gain
positive camber as they are deflected upward. The purpose of such a design is to reduce
the cornering power of the front end relative to the rear end, so that the car will
understeer in steadily greater amounts up to the limit of adhesion. Understeer is
inherently a much safer and more stable condition than oversteer, and thus is preferable
for cars intended for the public.

Since most independent suspensions are
designed so that the camber varies as the wheel moves up and down relative to the chassis,
the camber angle that we set when we align the car is not typically what is seen when the
car is in a corner. Nevertheless, it's really the only reference we have to make camber
adjustments. For competition, it's necessary to set the camber under the static condition,
test the car, then alter the static setting in the direction that is indicated by the test
results.

The best way to determine the proper
camber for competition is to measure the temperature profile across the tire tread
immediately after completing some hot laps. In general, it's desirable to have the inboard
edge of the tire slightly hotter than the outboard edge. However, it's far more important
to ensure that the tire is up to its proper operating temperature than it is to have an
"ideal" temperature profile. Thus, it may be advantageous to run extra negative
camber to work the tires up to temperature.

(TOP RIGHT) Positive
camber: The bottoms of the wheels are closer together than the tops. (TOP LEFT) Negative
camber: The tops of the wheels are closer together than the bottoms. (CENTER) When a
suspension does not gain camber during deflection, this causes a severe positive camber
condition when the car leans during cornering. This can cause funky handling. (BOTTOM)
Fight the funk: A suspension that gains camber during deflection will compensate for body
roll. Tuning dynamic camber angles is one of the black arts of suspension tuning.

TESTING IS IMPORTANT

Car manufacturers will always have
recommended toe, caster, and camber settings. They arrived at these numbers through
exhaustive testing. Yet the goals of the manufacturer were probably different from yours,
the competitor. And what works best at one race track may be off the mark at another. So
the "proper" alignment settings are best determined by you-it all boils down to
testing and experimentation.

John Hagerman is a mechanical engineer
who works for the U.S. Army as a vehicle test engineer at the Aberdeen Proving Grounds in
Maryland. John started autocrossing at the age of 16 in a Triumph Spitfire and switched to
road racing a few years later. Lately, he has been playing with a Sports 2000.